Sampel Conditioning System for Analyzer

Basic Concept and Construction of SCS

Sample conditioning systems are comprised of all the components necessary to extract a representative sample and to condition the sample for measurement by the analyzer. The design of the total sample conditioning system must be engineered for each specific application. The sample conditioning system assembly tends to be complex (actually small chemical processes with control systems) and is often the least reliable segment of an analyzer system. It is important that all factors that influence an overall system and the operation of an analyzer be given thorough consideration. Such factors as unknown process conditions, process upsets, and contaminants can lead to poor designs.

Primary Conditioning System

Installation of primary conditioning systems at the sample tap typically allows for more reliable operation of extremely difficult samples that cannot be transported easily to a remote sample conditioning system. Remote pressure let-down and vaporizer regulator stations are another type of primary conditioning system. These stations must be mounted near the sample tap to flash liquid samples and reduce lag times by controlling the sample pressure before transport to the analyzer location. Insulation of a heated enclosure may be required on these installations if the sample dewpoint temperature cannot be maintained.

Sample Probe

Sample probes should be used because they provide a more representative sample due to the higher flow rates away from process pipe walls.

Note: Process lines smaller than 2 in. usually do not require a sample probe. If a probe is required for small diameter lines, the line may be swaged up to 3 in. or larger with a spool piece to allow a probe to be inserted.

Sample probes significantly reduce wall contaminates and particulates and act as a first stage of filtration and conditioning. Special design considerations are required when process velocities are excessive (which may require thickwalled probes and in high temperature applications). Corrosive service or high temperatures may require special alloys or materials. Flanged thermowells are often used as sample probes by merely cutting the end off at a 45° angle. The probe is positioned 180 degrees from the process flow to minimize particulate matter in the extracted sample.

  Transport Line

In some installations, where the analyzer and sample system must be located at a distance from the sample point, the sample transport time must be considered. The time necessary to transport the sample to the analyzer can in some cases contribute the largest share to the overall system dead-time or turnaround time. Turnaround time is defined as the total time of sample system lag, dead-time, and the analyzer response time. A convenient means for reducing this transport lag is the use of a fast loop between the process equipment, an intermediate location (commonly just outside the analyzer shelter), and a low pressure return point in the process where the sample can be returned. Sample transport time, or lag time, is a function of the sample line length and diameter, the absolute pressure in the line, and the sample flow rate. Sample transport time is simply the total volume of the sample line divided by the average flow rate in the line. The lag time can be calculated with the aid of the following equations:

SAMPLE CONDITIONING SYSTEM Design

The functions performed by the sample conditioning system as the interface between the process and the process analyzer include the following:

• Taking and delivering a representative sample from the process.
• Transporting the sample from the sample tap to the analyzer and from the analyzer to the waste disposal and/or process return point.
• Conditioning the sample by adjusting the pressure, temperature, flow rate, filtering, and phase maintenance of the sample as required to make the sample compatible to the process analyzer.
• Sample-stream switching and calibration switching into the analyzer.
• Design incorporates capability for ease of maintenance, cleaning, or (when needed) flushing the entire system.

The complete stream composition of all the components and contaminants must be considered. Some contaminants, such as solids or entrained liquids in a gaseous stream, may have to be removed by the sample system. The process conditions and range of all components during normal and especially abnormal conditions (such as startup, shutdown, rate change, and so forth) must be considered for the analyzer application and sample system design.

Analyzer System Package Photo

Referenced Documents

• API RECOMMENDED PRACTICE 555 SECOND EDITION, NOVEMBER 2001 ANSI/API RP 555-2001
• Process Stream Analysis (Analyzer Sample Systems), A Review of Basic Design Consideration. (Emerson)